1. Field of the Invention
The present invention relates to a control system of a machine tool, more particularly relates to a control system of a machine tool which is capable of changing a speed of feed motion of a spindle.
2. Description of the Related Art
In various fields of machine tools, there is a widely used control method for controlling an operation of a feed axis so that a load applied to the spindle is decreased when a detected temperature of a drive device for a spindle (motor, amplifier, etc.) approaches a maximum allowable temperature. For example, JP2000-271836A proposes a control system which uses results of simulation of an amount of heat generation during fast feed or cutting feed of a feed axis so as to calculate the amount of heat generation during program execution and change a time constant of acceleration/deceleration of a feed operation so that the calculated value is equal to or less than an allowable value. Further, WO2005/093942A proposes a control system which subtracts copper loss and iron loss from a rated loss of a permanent magnet type motor, deems the obtained value as an allowable loss of the motor, and prevent overload of the motor by supplying the motor with a d-axis current corresponding to the obtained value. Further, JP2003-005836A proposes a drive device with a self-protection function, comprising a temperature sensor which detects a temperature of a servo driver (that is, amplifier), and a controller which lowers output of a drive part in accordance with the detected temperature of the temperature sensor.
In this way, a control method for adjusting a load applied to a spindle motor in accordance with a detected temperature of the spindle motor has been known from the prior art. However, a rise rate of the motor temperature fluctuates in accordance with the load applied to the motor at a current point of time, and therefore it is not possible to determine how much time is left before overheating of the motor with the current load being continued even if the prior arts of JP2000-271836A, WO2005/093942A, and JP2003-005836A are used. For this reason, in order to prevent overheating of the motor in the prior arts of JP2000-271836A, WO2005/093942A, and JP2003-005836A, it was necessary to adjust the load applied to the motor on the assumption of a large rise rate of the motor temperature even when the detected motor temperature was relatively low. That is, when the prior arts of JP2000-271836A, WO2005/093942A, and JP2003-005836A are used, it is necessary to conservatively control the operation of the motor, and therefore it is not possible to utilize the capacity of the motor to the maximum extent.
In relation to this, JP2014-156005A proposes a control system with the function of estimating from the current motor temperature and detected current value, the remaining time until a motor would overheat in the case where the current load is continued. However, the control system of JP2014-156005A only displays on a display device, the estimated value of the remaining time until overheating, and therefore the user had to adjust the load applied to the motor in consideration of the displayed remaining time so as to prevent overheating of the motor.
A control system of a machine tool which is capable of reliably preventing a spindle motor from overheating has therefore been sought.